The present invention generally relates to measurement of the distance of a shaft from the bottom of a vessel and the amount by which the shaft is offset from the center of the vessel. More particularly, the present invention relates to the precise measurement of shaft height and shaft offset in vessels employed in dissolution testing systems.
In the pharmaceutical industry, dissolution testing and analysis is required to be performed on samples taken from batches of tablets or capsules manufactured by pharmaceutical companies in order to assess efficacy and other properties. Dissolution analysis by automated means has become popular for increasing throughput and improving accuracy, precision, reliability, and reproducibility. Automation also relieves the tedium of manually performing a variety of requisite procedures, including: handling and delivering dosage units such as capsules and tablets; monitoring dissolution system parameters; manipulating the shafts carrying the agitation paddles or sample baskets; recording, displaying and printing accumulated data and test results; and cleaning and filtering the vessels employed in such procedures.
Despite the benefits accruing from automation, validation of the procedures employed in dissolution testing and analysis remains a critical consideration. A typical dissolution test requires, among other things, that a rotatable shaft equipped with a paddle or basket be properly positioned in the center of, and properly located a specified distance from the bottom of, a dissolution test vessel prior to conducting the test. The USP has promulgated guidelines for the pharmaceutical industry which are enforced by the FDA. Under USP 24, General Chapters, Dissolution (711), the shaft must be positioned such that its centerline is not more than 2 mm at any point from the vertical axis of the vessel, and such that the paddle or basket (typically mounted to the lower end of the shaft) be positioned at 25 mm xc2x12 mm from the bottom of the vessel.
Various hand-held devices have been utilized to carry out the measurements required to determine whether a shaft is positioned in a dissolution test vessel in compliance with the above-cited guidelines. Rulers, machinist calipers and micrometers, and pass/fail fixtures typify such devices and are known to persons skilled in the art. It is readily apparent to such skilled persons that operation of these devices requires a great deal of manual handling, with critical specifications largely determined by sight and feel. Conventional shaft measurement devices therefore engender an unacceptably high risk of error. There accordingly exists a long felt need for improved apparatus and methods for determining the position of a shaft installed in the vessel of a dissolution testing station.
According to one embodiment of the present invention, an apparatus is mountable over a vessel in which an elongate member is operable, and is adapted for determining the position of the elongate member in relation to the vessel. The apparatus comprises a mounting component for mounting the apparatus over an open end of the vessel, linearly displaceable plunger, a transducer. The plunger and the transducer are supported by and disposed below the mounting component. The transducer is disposed in operative relation to the plunger, and is adapted for sensing a displacement of the plunger and for producing an electrical signal indicative of the displacement of the plunger.
According to another embodiment of the present invention, the plunger is radially displaceable inwardly toward and outwardly from a central axis of the vessel, and is biased toward the central axis. A plurality of radially displaceable plungers and a plurality of corresponding transducers can be provided.
According to yet another embodiment of the present invention, the plunger is axially displaceable along a direction generally parallel in relation to a central axis of the vessel, and is biased away from the mounting component.
According to still another embodiment of the present invention, the apparatus comprises a mounting component for mounting the apparatus over an open end of the vessel, a linearly displaceable first plunger, a first transducer disposed in operative relation to the first plunger, a second plunger, and a second transducer disposed in operative relation to the second plunger. The first plunger, first transducer, second plunger, and second transducer arc supported by and disposed below the mounting component. The second plunger is linearly displaceable along a direction generally transverse to that of the first plunger. The first transducer is adapted for sensing a displacement of the first plunger and for producing an electrical signal indicative of the displacement of the first plunger. The second transducer is adapted for sensing a displacement of the second plunger and for producing an electrical signal indicative of the displacement of the second plunger.
According to a further embodiment of the present invention, the apparatus comprises a mounting assembly, a lateral plunger, a lateral transducer operatively disposed with respect to the mounting assembly and to the lateral plunger, a vertical plunger and a vertical transducer operatively disposed with respect to the mounting assembly and to the vertical plunger. Thc lateral plunger is slidably mounted to the mounting assembly and includes means for biasing the lateral plunger radially outwardly. The lateral transducer is adapted to encode positions of the lateral plunger and to produce an electrical signal proportional to a change in position resulting from the displacement of the lateral plunger. The vertical plunger is slidably mounted to the mounting assembly, and includes means for biasing the vertical plunger downwardly with respect to the mounting assembly. The vertical plunger further includes an upper end positions of the vertical plunger and to produce an electrical signal proportional to a change in position resulting from displacement of the vertical plunger.
The present invention also provides a method for determining the location of an elongate member in relation to a vessel in which the elongate member extends, and includes the following steps. A measurement apparatus is provided that comprises a mounting component, a linearly displaceable plunger supported by the mounting component, and a transducer supported by the mounting component. The mounting component is placed over an open end of the vessel, such that the plunger and the transducer are suspended within the vessel. The elongate member is inserted through an aperture of the mounting component and into the vessel until the elongate member reaches an operating position. At this time, the elongate member is in contact with the plunger. After the elongate member reaches the operating position, the transducer measures a magnitude by which the plunger is displaced and produces an electrical signal indicative of the magnitude of displacement. In one aspect of the method, the displacement of the plunger occurs along a radial direction away from a central axis of the vessel. In another aspect, the displacement of the plunger occurs along an axial direction in generally parallel relation to the elongate member.